1. Field of the Invention
The present invention relates to an electrode forming method for a surface acoustic wave device (hereinafter referred to as a SAW device), and more particularly, it relates to a method of forming electrodes on a piezoelectric substrate structuring a SAW device through a thin film forming method.
2. Description of the Background Art
A SAW device utilizing a surface acoustic wave, which is generally structured by forming an interdigital transducer made of comb electrodes and grating electrodes of metal strips on a surface of a substrate having piezoelectricity, is widely applied to a television receiver or a video tape recorder. A material for forming the electrodes of such a SAW device is generally prepared from aluminum (Al), ordinarily in the form of amorphous polycrystalline Al.
Further, the SAW device is widely applied to a transmission/receiving element or a resonator for a high frequency domain in recent years, and expected as a filter for a radio frequency band pass filter of a portable device for mobile communication, for the purpose of miniaturization and weight reduction.
Such a SAW device is used at a low applied power level of about 1 mW when the same is applied to a television receiver or a video tape recorder, while a high voltage level signal is applied to a SAW device which is employed for mobile communication, particularly for the purpose of transmission. For example, extremely high power of about 20 mW is applied to a SAW filter for a cordless telephone (The 14th Symposium with respect to the fundamentals and applications of ultrasonic electronics on Dec. 9, 1993 held in Japan). Therefore, high stress which is caused by a surface acoustic wave is applied to the electrodes (Al electrodes), to result in migration in the electrodes. Such migration resulting from stress is called stress migration. This stress migration causes electrical shorting, increase in insertion loss and reduction in quality factor of the resonator, leading to reduction in performance of the SAW device.
In order to solve this problem, there has been proposed a SAW device employing as an electrode material an Al film or an Al alloy (e.g., Al-Cu alloy) film whose (111) plane is parallel to the substrate surface and the orientation axis is [111] (Japanese Patent Laying-Open No. 5-183373 (1993)).
In the aforementioned Al film or Al alloy film whose (111) plane is parallel to the substrate surface, however, resistance against stress migration is improved but this effect is not necessarily sufficient. Under the present circumstances, therefore, awaited is an electrode forming method which enables formation of electrodes having higher resistance against stress migration.